1. Field of the Invention
The present invention relates to a scanning exposure apparatus, used in a photolithography process in the manufacture of, e.g., a semiconductor element, a liquid crystal display element, a thin-film magnetic head, or the like, for transferring a pattern on a mask onto a substrate by synchronously moving the mask (or reticle) and the substrate and, more particularly, to a scanning exposure apparatus suited for a case wherein light having a high spatial coherency (e.g., harmonics of a KrF or ArF excimer laser, YAG laser, or the like) is used.
2. Related Background Art
In the photolithography process for the manufacture of semiconductor elements, a reduction projection exposure apparatus (stepper) adopting a step-and-repeat method for transferring a pattern on a mask or reticle (to be generally referred to as a "reticle" hereinafter) onto a semiconductor wafer coated with a photosensitive material (photoresist) via a projection optical system is used. In a stepper of this type, in order to improve the resolution by decreasing the wavelength of exposure light, it has been proposed to use, as exposure light, laser light in a far (or deep) ultraviolet range, e.g., harmonics or the like of a KrF or ArF excimer laser, a YAG laser, or an argon laser. At present, a stepper using the KrF excimer laser has been put into practical use, and is operating in manufacturing lines.
Laser light generally has a high spatial coherency (coherence) and forms a speckle pattern (interference fringes) on a reticle. As a result evenness of the illuminance on the reticle and wafer is impaired. In view of this, problem as disclosed in, e.g., U.S. Pat. No. 4,619,508 and Japanese Laid-Open Patent Application No. 1-259533 (corresponding to U.S. Pat. No. 5,307,207 (Mar. 13, 1989)), a pivot mirror is arranged at the light source side of a fly-eye lens in an illumination optical system to change the incident angle of laser light onto the fly-eye lens for every one to several pulses. With this arrangement, the interference fringes sequentially move on the reticle during exposure. Therefore, the evenness of the illuminance on the reticle or wafer, i.e., the evenness of the exposure amount, can be improved.
Recently, it is required to widen the image field of the projection optical system and to improve its resolution in correspondence with an increase in size and a decrease in line width of semiconductor elements. However, it is very difficult in terms of design and manufacture to realize both the high resolution and wide field of the projection optical system. Under the circumstances, a scanning exposure apparatus as disclosed in, e.g., U.S. Pat. Nos. 4,747,678, 4,924,257, and 5,194,893 has been the subject of much attention. In such an apparatus, a pattern on a reticle is transferred onto a wafer by illuminating only a local area on the reticle with light and synchronously moving the reticle and wafer. The scanning exposure apparatus can transfer a large-area pattern image onto the wafer even if the image field of the projection optical system is small, and can relatively easily improve the resolution of the projection optical system.
In the scanning exposure apparatus, since the reticle and wafer are synchronously scanned, the relationship between the moving amount (pitch) of the reticle and wafer and the pitch (in the scanning direction) of interference fringes in the illumination area between pulse emissions changes depending upon the scanning speed of the stage (i.e., an optimal exposure amount of the wafer). Therefore, when the scanning exposure apparatus uses light having a high spatial coherency as exposure light, it is difficult to reduce exposure amount unevenness caused by interference fringes even when the above-mentioned pivot mirror is used.